1. Field of the Invention
The present invention relates to semiconductor integrated circuits, and more particularly to a circuit which generates internal power supply voltage supplied to sense amplifier.
2. Description of the Background Art
Conventionally, a semiconductor memory device has been operated by an internal power supply voltage which is generated by an incorporated internal voltage-down converter and is lower than the externally supplied power supply voltage in order to maintain its compatibility with conventional devices and also reduce power consumption.
FIGS. 26 and 27 are circuit diagrams showing configurations of conventional voltage-down converters shown in FIG. 69 and the like of Japanese Patent Laying-Open No. 3-214669. The voltage-down converter shown in FIG. 26 generates a power supply voltage for array Vccs and includes a node NVccs for the power supply voltage for array, a differential amplifier configured of transistors 1, 2, 5, 6 and 7, a differential amplifier configured of transistors 9, 10, 12, 13 and 14, a transistor 3, driver transistors 4 and 11, and a capacitor 8.
In this voltage-down converter, the differential amplifier configured of transistors 1, 2, 5, 6 and 7 compares power supply voltage for array Vccs with a reference voltage Vref (e.g. 2.0V). When power supply voltage for array Vccs is lower than reference voltage Vref, transistors 4 is turned on to connect node NVccs for the power supply voltage for array to an external power supply voltage (e.g. 3.3V) node NVcc and thus raise power supply voltage for array Vccs to reference voltage Vref. Thus, power supply voltage for array Vccs is maintained at the level of reference voltage Vref.
The differential amplifier configured of transistors 9, 10, 12, 13, and 14 operates in a similar manner to the differential amplifier described above, although transistors 9, 10, 12, 13 and 14 are smaller in size than transistors 1, 2, 5, 6 and 7 and thus provide less power consumption.
Thus, when the semiconductor memory device is in a stand-by state, an activation signal Vdce is controlled to attain a low (L) level and the differential amplifier configured of transistors 9, 10, 12, 13 and 14 only operates to reduce the power consumption of the semiconductor memory device in the stand-by state.
The voltage-down converter shown in FIG. 27 generates a voltage VBL the magnitude of which is half that of power supply voltage for array Vccs, and includes transistors 38-43.
FIG. 28 is a circuit diagram showing the configuration of a memory cell array portion of a conventional semiconductor memory device. As shown in FIG. 28, the memory cell array portion includes bit lines BL0 and BLn, inverted bit lines /BL0 and /BLn, a p-type sense amplifier configured of transistors 16-19, a transistor 15 supplying a power supply voltage for array Vccs to the p-type sense amplifier, an n-type sense amplifier configured of transistors 20-23, a transistor NT1 supplying a ground voltage to the n-type sense amplifier, a bit line equalization circuit configured of transistors 24-29, word lines WL0 and WLm, memory cells MC0 to MC3, and parasitic capacitances CB0 to CB2n+1. Memory cell MC0 includes a capacitor 34 and a transistor 30, memory cell MC1 includes a capacitor 35 and a transistor 31, memory cell MC2 includes a capacitor 36 and a transistor 32, and memory cell MC3 includes a capacitor 37 and a transistor 33.
Data of high (H) or L held in memory cells MC0 to MC3 correspond to power supply voltage for array Vccs or 0V applied to capacitors 34, 35, 36 and 37. The pairs of bit lines are precharged to attain an intermediate voltage (1/2Vccs, i.e., voltage VBL) in reading the data held in memory cells MC0 to MC3. Then, when word line WL0 is selected, for example, to connect capacitors 34 and 35 in memory cells MC0 and MC1 to bit lines BL0 and BLn. If capacitors 34 and 35 hold data of H, the voltage of bit lines BL0 and BLn exceeds voltage VBL. If capacitors 34 and 35 hold data of L, the voltage of bit lines BL0 and BLn drop below voltage VBL. Meanwhile, the voltages of inverted bit lines /BL0 and /BLn remain at voltage VBL and potential differences are caused between paired bit lines. The potential differences are amplified by the sense amplifiers to determine the data held in memory cells MC0 and MC1.
A data read operation when memory cells MC0 and MC1 hold data of H will now be described with reference to the timing charts shown in FIGS. 29A-29G. The state prior to time t1 is a stand-by state, and a bit line equalization signal BLEQ is at a high level, as shown in FIG. 29F, and paired bit lines BL0 and /BL0, and BLn and /BLn are precharged via transistors 24-29 to attain voltage VBL. When word line WL0 is activated to attain a high level at time t2, as shown in FIG. 29C, transistors 30 and 31 are turned on to allow the data of H to be transmitted on bit lines BL0 and BLn and the voltage of bit lines BL0 and BLn exceeds voltage VBL, as shown in FIG. 29B. Meanwhile, the voltage of inverted bit lines /BL0 and /BLn remains at voltage VBL and a potential difference is thus created between paired bit lines BL0, /BL0, BLn and /BLn. At time t3, sense amplifier activation signals SEP and SEN respectively attain a low level and a high level, as shown in FIGS. 29D and 29E, to turn on transistors 15 and NT1 and activate both the p-type sense amplifier configured of transistors 16-19 and the n-type sense amplifier configured of transistors 20-23. Thus, at time t4, the voltages of bit lines BL0 and BLn are raised to power supply voltage for array Vccs and the voltages of inverted bit lines /BL0 and /BLn are lowered from voltage VBL to the ground voltage. When the voltages of bit lines BL0 and BLn recover to the power supply voltage Vccs for array in the stand-by state, i.e. 2.0V, at time t5, the power supply voltage Vccs for array in the stand-by state, i.e. 2.0V, is rewritten into capacitors 34 and 35 as a holding voltage for capacitors 34 and 35 which have been lowered in reading the data.
For conventional semiconductor memory devices, however, the power supply voltage for array Vccs when the sense amplifiers operate drops from a predetermined level (2.0V) when the voltages of bit lines BL0 and /BLn are raised from voltage VBL at time t4, as shown in FIG. 29A. Then the voltage-down converter shown in FIG. 26 operates and the dropped power supply voltage for array Vccs thus recovers to the predetermined level, although the recovery requires a certain period of time. In other words, bit lines BL0 and BLn each attain a high level by supply of power supply voltage for array Vccs and accordingly the time for amplifying a potential difference of a pair of bit lines depends on the operating time of the voltage-down converter shown in FIG. 26. Thus, it has been difficult for conventional semiconductor memory devices to rapidly rewrite data into memory cells.
By contrast, Japanese Patent Laying-Open No. 6-215571 discloses the semiconductor memory device shown in FIG. 30. As shown in FIG. 30, the semiconductor memory device includes a pair of bit lines BL and /BL, word lines WL1 and WL2, a precharger circuit PC, input/output data lines I/O and /I/O, a sense amplifier, a node NVccs for power supply voltage for array, transistors Q2 and Q15, a voltage-down converter BVDL connected to node NVccs for power supply voltage for array, and a parasitic capacitance C1 connected to node NVccs for power supply voltage for array, wherein voltage-down converter BVDL includes transistors Q51-Q56.
An operation of the semiconductor memory device will now be described.
Parasitic capacitance C1 is charged to attain an external power supply voltage Vcc when transistor Q2 is turned on while transistor Q15 is turned off and the sense amplifier is inactivated. Then, when transistor Q15 is turned on and the sense amplifier is activated, the electric charge for charging a bit line is supplied from parasitic capacitance C1 and voltage-down converter VBDL. Thus, data is rewritten more rapidly in this semiconductor memory device than in the conventional semiconductor memory device described above. In the semiconductor memory device thus configured, however, it is not until the voltage at node NVccs for power supply voltage for array drops below a reference voltage VR that a differential amplifier of current mirror type (configured of transistors Q51-Q55) included in voltage-down converter BVDL turns on a driver transistor Q56 to accelerate charging the bit line. Since driver transistor Q56 has a large gate width and hence a large gate capacitance, it typically requires approximately 10 ns for the differential amplifier of current mirror type to lower the gate potential of driver transistor Q56 to completely turn on driver transistor Q56. Thus, even a semiconductor memory device thus configured cannot overcome the disadvantage that the time required for charging a bit line depends on the response speed of a differential amplifier of current mirror type.